What is meant by heuristic optimisation?Discuss the major heuristics that are applied during query optimisation.                                                                                       

Ans:   In heuristic opimization, heuristics are employed to decrease the cost of optimization in place of analyzing the number of dissimilar plans to find out the optimal plan. For instance A analyzing optimizer would make use of the rule 'perform selection operation as early as possible' without finding out whether the cost is decreased by this transformation.  Heuristics approach generally assists to reduce the cost but not always. The heuristics which are applied during query optimization are:

a)      Pushes the selection and projection operations down the query tree

b)      Left-deep join trees- convenient for pipelined evaluation

c)       Non- left-deep join trees